The present invention relates to a method for making a raw board for use in printed circuits.
Hitherto, raw boards for use in printed circuits had been prepared by heating and pressing copper foil with an adhesive, if desired, onto a plastic sheet such as phenol resins and epoxy resins.
In such a case, copper foil having a thickness of at least 30.mu. or so has generally been used because comparatively thin copper foil cannot be employed in smooth state due to the fact that the foil is liable to be broken or creased at the preparing or cutting stage of same prior to the heating and pressing stages at which the copper foil is heat-pressed onto a plastic sheet.
However, the use of the copper foil 30.mu. or so in thickness results in the occurrence of a side-etching phenomenon at the etched portions in the case where a given circuit is printed and then unnecessary copper portions are dissolved and removed by an etching operation, thus creating such problems that the breaking of the circuit occurs or a detailed circuit cannot be obtained.
U.S. Patent Application, Ser. No. 886826 filed Mar. 15, 1978 by J. Sato discloses a process for preparing a raw board for use in printed circuits which does not necessitate adhesion of a copper foil. The inventors of the present invention further undertook to improve the qualities of the raw board for use in printed circuits since room for improvement was left in the peeling strength and repeated folding resistance of the copper layer deposited on the surface of the raw board.
With regard to the peeling strength of the copper layer, it was found that far better peeling strength than a standard value required could be obtained by electroplating copper on a plastic substrate, which had adhered copper thereon, at 40.degree.-90.degree. C. with a copper sulfate electroplating bath containing a thiourea-including-additive which had hitherto been used as a surface glazing agent.
Although the copper layer deposited on the raw board thus obtained has excellent resistance to repeated-folding test (hereinafter referred to as "repeated folding resistance") as compared with that of a conventional raw board for use in printed circuits, there exists in the art a demand for obtaining further superior repeated-folding-resistance.
The inventors of the present invention carried out various experiments in wide range to improve the repeated-folding-resistance of the raw board while maintaining the peeling strength at a required level. As a result, it was found that the improved resistance may be obtained by the steps of (A) electroplating copper onto a plastic substrate at a given temperature with the above mentioned bath containing a thiourea including type additive and thereafter (B) further electroplating copper thereon with a conventional copper sulfate bath not containing the above additive. In addition, it was further found that the combination of the peeling strength and the repeated-folding-resistance may be optionally varied by controlling the ratio of the electroplated thickness of the copper layer obtainable with the bath containing the thiourea including additive to that of the copper layer obtainable with a bath not containing the aforementioned additive.